The present invention is directed to a system for detecting air leaks in vessels and containers in which a vacuum is formed. Leak-detection in vessels and containers used in a vacuum environment is well-known, such detection utilizing the detection of leaks of helium previously pumped into the vacuum vessel expressly for the purpose of detecting potential leaks. Helium leak-detection is possible owing to the lightness of the gas and its concomitant small molecular size, allowing detection of even the smallest hole or tear. The leaking helium is detected by a simple and conventional mass spectrometer that is designed to detect only helium gas. However, for such a mass spectrometer to operate effectively, such must be evacuated to high vacuum, which allows the probe, or leaking, helium to be drawn into the detecting chamber of the mass spectrometer, and detected by the helium-sensing head. However, since the probe helium gas is traveling from an essentially atmospheric pressure environment to one of very high vacuum in the detecting chamber of the mass spectrometer, complicated and expensive variable-leak throttling valves are required to allow for the introduction of the higher pressure helium into the detection chamber of the mass spectrometer so that the helium sensing head thereof is not adversely affected by a rise in pressure. The complex throttling valve allows for such introduction. In order to sustain the high vacuum in the detection chamber of the mass spectrometer, a high vacuum pump is required. The original pump used was a high-vacuum oil diffusion pump. Since oil vapors from the pump would contaminate the mass spectrometer sensing head, liquid nitrogen traps were employed to freeze out the oil vapors before reaching the sensing head. The use of liquid nitrogen was and is a difficult and costly process, as well as requiring the maintenance of an adequate supply.
An alternative to the use of oil diffusion pumps has been the use of turbomolecular pumps, which has only been commonplace within the last few years, owing to the refinement and development of these kinds of pumps. The turbomolecular pump is essentially an axial-flow molecular turbine having a plurality of alternately-arranged slotted rotating blades and stationary stator blades, with the relative velocity between the two sets of blades making it highly probable that a gas molecule will be transported from the pump inlet to the pump outlet. Since the gas is compressed only slightly by each stage, a series of such blades are required to achieve an effective compression ratio and workable and effective pressure differential. The turbomolecular pump deals with molecular flow, with compression achieved via momenta-transfer from the high-speed rotating blades to the gas molecules. The operating exhaust pressure is in the range of about 30 millitorr, which extremely low pressure, like the oil diffusion pump, has required complex and expensive throttling valves to allow for the introduction of the probe helium gas into the sensing probe chamber, as explained above. The use of the turbomolecular pump, however, was an advancement in that it more effectively prevented the simultaneous introduction of oil vapors, though such was not completely eliminated as a problem, since an oil-sealed mechanical pump was required in series with the turbomolecular pump in order to achieve and sustain such extremely low operating pressures. The additional advantage provided was the fact that turbomolecular pumps will pump heavy gases more readily and easily than lighter gases, such as helium, so that the technique of "Back-Diffusion" or "Counter-Flow" was developed using the turbomolecular pumps, by which the probe helium gas to be detected was introduced at the outlet of the exhaust of the turbomolecular pump, with the probe helium diffused rearwardly through the turbomolecular pump until it reached the sensor probe of the mass spectrometer, the heavier air molecules having been "filtered out" or selectively eliminated by this process. The laws governing such diffusion are based on molecular flow and statistical thermodynamics. However, as stated above, complex throttling valves are still required, owing to the extremely low exhaust pressure at the pump outlet.
The present invention is directed to a considerably improved helium-leak detection system by which the detection-sensitivity is increased, oil-vapor diffusion is completely obviated, and the use of a throttling valve is eliminated. The present invention has achieved such a remarkable and improved leak-detection system by the use of the relatively recently-developed molecular drag pump instead of the turbomolecular pump above-described. The molecular drag pump, which includes the Gaede molecular drag pump, as well as the modern and greatly advanced version of the old Holweck pump, compresses a gas along the axial flow-direction, in contradistinction to the turbomolecular pump which imparts compression transversely to the flow-direction. In the disc-type molecular drag pump, such compression is achieved by a rotating rotor in which is formed a series of precisely-aligned and formed spiral grooves that cooperate with several parallel helical grooves formed in the stator. The use of the molecular drag pump in a leak-detection system has allowed for the above-noted advantages and improvements as compared to the turbomolecular pump systems, since the outlet or exhaust pressure of the molecular drag pump is of the order of one-thousand times that of the turbomolecular pump: 30 torr as compared with 30 millitorr.